1. Field of the Invention
The present invention relates to a plasma etching method for a silicon dioxide (SiO.sub.2) film and more particularly, to a plasma etching method for forming a penetrating hole in a doped or undoped silicon dioxide film equipped with a mask thereon using a fluorocarbon (C.sub.n,F.sub.m)-based etching gas, which is preferably applied to semiconductor device fabrication.
2. Description of the Prior Art
In recent years, the design rule has been becoming lower and lower with the progressing integration degree of semiconductor integrated circuit devices (ICs). On the other hand, to planarize the surface of a SiO.sub.2 -based interlayer dielectric layer, the Chemical Mechanical Polishing (CMP) process has been being popularly used. Therefore, there has been a tendency that the interlayer dielectric layer becomes thicker.
Under such circumstance, contact holes formed through the interlayer dielectric layer tend to be not only smaller in size or diameter but also larger in depth. This means that the contact holes need to have not only small diameters but also high aspect ratios, where the aspect ratio is defined as a ratio of the depth of the contact holes to the diameter thereof.
Conventionally, as a microscopic processing method for SiO.sub.2 films, plasma etching has been known and widely used in the field of semiconductor device fabrication. In plasma etching, for example, a fluorocarbon (C.sub.n F.sub.m)-based gas is introduced into a reaction chamber as an etching gas, where n is an integer ranging from 1 to 6 and m is an integer ranging from 4 to 14. Then, a specific high voltage is applied across electrodes to cause a glow discharge in the reaction chamber, thereby generating a plasma in the chamber. The molecules of the (C.sub.n F.sub.m)-based gas that has been introduced into the chamber collide with energetic electrons existing in the plasma, thereby inducing dissociation of the (C.sub.n F.sub.m)-based gas. In this case, typically, neutral dissociation and ionic dissociation of the (C.sub.n F.sub.m)-based gas occur simultaneously.
In the reaction chamber, active or reactive species, i.e., adherent and neutral radicals and molecules of fluorocarbon C.sub.n F.sub.m, which are generated by the neutral dissociation, tend to be deposited on a SiO.sub.2 layer to be etched, thereby forming a film of fluorocarbon polymer hereon. At the same time as this deposition, energetic ions generated by the ionic dissociation tend to collide with the deposited fluorocarbon polymer film, thereby vaporizing the fluorocarbon polymer film and the underlying SiO.sub.2 film. Thus, the SiO.sub.2 film is etched as desired.
When the SiO.sub.2 film to be etched is covered with a patterned masking film to form a penetrating hole with a high aspect ratio in the SiO.sub.2 film, the etch selectivity of the SiO.sub.2 film to the masking film needs to be as high as possible. If this necessity is accomplished, the SiO.sub.2 film is selectively etched to have a desired topography according to the pattern of the masking film.
To realize a high etch selectivity of the SiO.sub.2 film to be etched to the overlying masking film, various techniques have been developed.
An example of the known, developed techniques is to add gaseous carbon monoxide (CO) to a fluorocarbon-based etching gas. This technique is disclosed in a document written by E. Ikawa, Technical Proceedings Semicon/Japan 1993, pp. 405-411, which was entitled "Subjects on Dry Etching Technology for Coming Generation Device Process" and published in 1993.
In this conventional technique, addition of CO to the fluorocarbon-based etching gas increases the ratio of carbon (C) to fluorine (F), i.e., the (C/F) ratio, of the deposited fluorocarbon polymer film, thereby improving the sputtering or etching resistance property of the fluorocarbon polymer film. In other words, the etch rate of the fluorocarbon polymer film deposited on the masking film becomes low during the plasma etching process while the etch rate of the SiO.sub.2 film as an etching object is kept approximately unchanged. As a consequence, the etch selectivity of the SiO.sub.2 film to the masking film and the deposited fluorocarbon polymer film is increased.
Another example of the known, developed techniques is disclosed in the Japanese Non-Examined Patent Publication No. 8-92768 published in 1996. In this conventional technique, (i) a mixture of a C.sub.n F.sub.m -based gas and a neon (Ne) gas, (ii) a mixture of a CnFm-based gas, a Ne gas, and a Co gas, or (iii) a mixture of a C.sub.n F.sub.m.-based gas, a helium (He) gas, and a CO gas is used as the etching gas. A silicon nitride (Si.sub.3 N.sub.4) film is used as the masking or etching resist film.
In this conventional technique, due to the use of any one of the above mixtures (i), (ii) and (iii), the (C/F) ratio of the deposited fluorocarbon polymer film on the masking film is increased. As a result, the etch selectivity of the SiO.sub.2 film to masking film and the deposited fluorocarbon polymer film becomes higher.
With the above-described two conventional plasma etching techniques, however, the following problems will occur.
Specifically, to form a penetrating hole with a smaller diameter or size such as approximately 0.30 .mu.m or less in a SiO.sub.2 film, an extremely thin masking film needs to be formed by a deep ultraviolet (UV) resist material in view of an necessary patterning accuracy. In this case, the etching resistance of the masking film becomes lower with the decreasing thickness of the same.
Therefore, it is difficult to form the small-sized penetrating hole (i.e., approximately 0.30 .mu.m or less in diameter) with a high aspect ratio in the SiO.sub.2 film by simply increasing the C/F ratio of the deposited fluorocarbon polymer film on the masking film as disclosed in the above-described conventional plasma etching techniques.
In fact, the inventor has never known any report that a penetrating hole with a diameter equal to or less than 0.25 .mu.m and a high aspect ratio is formed in a SiO.sub.2 film using a patterned masking film.
Thus, to deal with the fabrication processes for next-generation semiconductor devices, there has been the strong need of realizing a plasma etching method to form a penetrating hole with a small diameter of approximately 0.3 .mu.m or less and a high aspect ratio in a SiO.sub.2 film.